MEXICO CITY — Mexican officials, scrambling to control a swine flu outbreak that has killed at least 16 people and possibly dozens more in recent weeks, shuttered schools from kindergarten to university for millions of young people in and around the capital on Friday and urged people with flu symptoms to stay home from work.

“We’re dealing with a new flu virus that constitutes a respiratory epidemic that so far is controllable,” Health Minister Jose Angel Cordova told reporters late Thursday, after huddling with President Felipe Calderón and other top officials. He said the virus had mutated from pigs and had at some point been transmitted to humans.

Mexico’s flu season is usually over by now, but health officials have noticed a significant spike in flu cases. The World Health Organization reported about 800 cases of flu-like symptoms in Mexico in recent weeks, most of them among healthy young adults, with 57 deaths in Mexico City and 3 in central Mexico.

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That is a worrying pattern because seasonal flus typically cause most of their deaths among infants and old people, while pandemic flus, such as the 1918 Spanish flu, often strike young, healthy people the hardest. Doctors believe that it is because young adults have more vigorous immune systems — which mount an assault on the new virus known as a “cytokine storm” — that may actually overwhelm the victim’s own lungs by causing inflammation and drawing in fluid.

It was clear that Mexican health officials were alarmed. Besides shutting classes, the government urged people to avoid large gatherings and to refrain from the common greetings of a hand shake or kiss on the cheek. City buses continued to operate but some passengers were seen wearing masks, and a cough or a sneeze by one passenger prompted others to relocate.